Early work by Siringo, Pucci and March (Phys. Rev. B, 37, 2491 (1988)) studied solid I2 under high pressure at T = 0. Their conclusion was that insulating crystalline I2 at low pressures eventually transformed into a molecular metal. This has subsequently been confirmed experimentally. Later studies by Weir et al. (Phys. Rev. Lett., 76, 1860 (1996)) on solid H2 under pressure point in the same direction, though in the solid phase the metallic state has still not been achieved at low temperatures. However, in the liquid phase, an insulating metallic transition has been proposed, as in solid iodine, again on the basis of experimental high-pressure studies. Here, attention is shifted to some polyatomic molecules, such as the 10-electron series H2O, NH3 and CH4. Particular attention is focused on the measured Hugoniots of the polyatomic molecular liquids